While forum posting and blog commenting may help drive traffic, if your contributions are of high quality, they are useless from the point of view of backlinks. Many forums, like SitePoint, make all user-submitted links nofollow, as do many blogs. Even where you can find forums or blogs which do not do this, the links you can get there are pretty worthless. Search engines put very little value on easily-obtainable, self-generated links.
Stevie D beat me to it; we can't both be wrong, can we?